1. Technical Field
This invention relates to a system of components to be hybridized adapted to a hybridization technique by melting of solder balls of the type referred to as "flip-chip". The invention also relates to a process for preparing these components and a hybridization process using these components.
The invention will be applied particularly in the electronic and optical fields for interconnection of components made of different materials.
For example, the invention is particularly useful for interconnection of silicon components with AsGa or InSb components.
For the purposes of this invention, a component may be an electronic component such as an electronic chip, an electronic or opto-electronic circuit support, or a mechanical component such as a cover or a physical magnitudes sensor.
In particular, the invention may be used for the manufacture of infrared detectors, the manufacture of lasers with a vertical cavity, or to transfer a matrix of AsGa read photodiodes onto an Si read circuit.
2. State of Prior Art
A distinction is made between two main techniques for hybridization of components by solder balls.
The first technique is called the "hybridization by melting technique" (or "flip-chip"), and uses balls of a meltable material, for example such as a tin and lead alloy (SnPb) or a tin and indium alloy (SnIn). This technique is shown in the drawing on FIGS. 1 and 2 in the appendix.
On these figures, references 10 and 12 refer to first and second components to be interconnected, respectively. The first component 10 comprises the first ball reception pads 14a, made of a material wettable by the solder balls material. Pads 14a are surrounded by an area 16a of material not wettable by the material in the balls.
Similarly, the second component 12 has second pads 14b also made of a material wettable by the balls material and surrounded by an area 16b of unwettable material.
The first and second pads 14a, 14b are associated to form pairs of pads, in locations complementary to the first and second components respectively. Thus the first and second pads in each pair of pads are located approximately opposite each other when the components to be hybridized, 10, 12 are placed in contact with each other as shown on FIG. 1.
It is considered that FIG. 1 shows the closed structure consisting of the two components at ambient temperature denoted Ta. Let L be the distance that separates the two pads on which the balls fit on each of the component.
Pads 14a of one of the components, in the event component 10, are equipped with balls 18 made of a meltable material. These balls 18 are designed to create a mechanical and/or electrical link between the pads in each pair of pads.
In order to do the hybridization, component 12 is moved onto component 10 in order to bring the pads 14b into contact with the corresponding balls 18. The entire structure is then heated up to or greater than the melting temperature of the balls so that balls 18 are soldered onto pads 14b.
All balls are thus soldered simultaneously onto their corresponding pads 14b on component 12. The structure shown on FIG. 2 is obtained after cooling. On this figure, each pad 14a of component 10 is mechanically and electrically connected to a corresponding pad 14b on component 12, by a solder ball 18.
The precision of the mutual positioning of components when component 10 is transferred to component 12 is not very critical. When the ball material melts, components 10 and 12 automatically align with each other under the effect of the surface tension of the material making up the molten balls.
In the hybridization procedure described above, all solders between the balls and reception pads are made at the same time. The hybridization by fusion process is particularly suitable for hybridization of a number of components, such as chips, on a reception component forming substrate. A high hybridization efficiency can be obtained for these structures.
FIGS. 1 and 2 illustrate the use of the hybridization by melting process for small components, and for components which have very similar coefficients of expansion.
When components to be hybridized have different coefficients of expansion, and particularly when there is a non-negligible distance between the different pads, the pads on the two components are no longer aligned when the structure temperature is increased to or above the temperature at which the ball material melts.
This situation is illustrated on FIG. 3 which shows components at the hybridization temperature Th. It is considered that the material in component 12 has a coefficient of expansion .alpha..sub.2 exceeding the coefficient of expansion .alpha..sub.1 of the component material 10. When the structure temperature is increased to the hybridization temperature Th, the pads 14a on component 10, and consequently balls 18, are separated by a distance L' such that: EQU L'=L(1+.alpha..sub.1 -.DELTA.T)
where L is the distance between the same pads 14a at ambient temperature Ta (FIG. 1), and .DELTA.T is defined by .DELTA.T=Th-Ta.
In the same way, the pads 14b on component 12 are separated by a distance L' at the hybridization temperature Th such that: EQU L"=L(1+.alpha..sub.2 -.DELTA.T)
In the special case shown on FIG. 3 in which each component only has two ball reception pads, the misalignment .DELTA.L between each pad 14b and each corresponding ball 18 is then such that: EQU .DELTA.L=L/2..DELTA..alpha...DELTA.T,
where .DELTA..alpha.=.alpha..sub.2 -.alpha..sub.1.
In some cases the misalignment between ball reception pads on substrates to be hybridized can be sufficient to compromise the hybridization operation.
For example, when substrate 10 is made of silicon (.alpha..sub.1 (Si)=2.10.sup.-6), and substrate 12 is made of gallium arsenate (.alpha..sub.2 (AsGa)=8.10.sup.-6), and when the distance between the balls 18 at an ambient temperature of 20.degree. C. is 2 cm, and when the hybridization temperature of the 60/40 tin-lead alloy balls 18 is of the order of 220.degree. C., the lateral offset between each pad 14b and each corresponding ball 18 is .DELTA.L=12 .mu.m.
For example, this order of magnitude is reached for components in the form of modules with 1000 aligned connection pads at a pitch of 20 .mu.m.
In this case, if the diameter of the wettable surface of connection pads is for example 12 .mu.m, only balls for which offset .DELTA.L is less than or equal to 6 .mu.m can be connected using the hybridization by melting process. Only these balls will be in contact with the reception pads on the second component.
In order to overcome these difficulties, there is a second technique for hybridization by solder balls. This second technique is referred to as the hybridization by pressure technique.
This technique starts with a structure similar to the structure shown on FIG. 1. Component 12 is transferred to component 10 by making the pads 14b coincide very precisely with the balls 18. Then, balls 18 are pressed firmly into contact with pads 14b in order to form a connection by thermo-compression, by exerting appropriate forces on components 10 and 12.
This operation may take place at a temperature below the ball material melting temperature, and in particular at a temperature close to ambient temperature.
Thus, by limiting temperature excursions .DELTA.T, the problem of a misalignment between pads 14b and balls 18, or pads 14a, does not arise, even when the components are made of materials with different coefficients of expansion.
However, the hybridization by pressure technique does have a number of disadvantages compared with the hybridization by melting technique.
For example, for hybridization by pressure, components to be hybridized must be aligned with very high precision. The self-alignment phenomenon described above does not occur at temperatures below the melting point of the ball material.
Furthermore, unlike hybridization by melting, hybridization by pressure is unsuitable for the simultaneous connection of a large number of components during the same operation.
Furthermore, much lower hybridization rates are possible with hybridization by pressure.
Finally, one purpose of this invention is to propose a system of components to be hybridized and a process for preparation and hybridization of components, which does not have the disadvantages or the limitations mentioned above.
In particular, one purpose of the invention is to propose a system of components to be hybridized and a hybridization process which does not have any misalignment problems at the hybridization temperature.
Another purpose is to propose a hybridization process enabling the simultaneous interconnection of a large number of components.
Another purpose of the invention is to propose a system of components to be hybridized and a hybridization process capable of automatically aligning components.